工作结束后，是否可以使线程加入"parallel for"区域? [英] Is it possible to make thread join to 'parallel for' region after its job?

问题描述

我有两个需要首先同时运行的工作:

I have two jobs that need to run simultaneously at first:

1)用于可并行化的循环

1) for loop that can be parallelized

2)可以用一个线程完成的功能

2) function that can be done with one thread

现在，让我描述一下我想做什么.

Now, let me describe what I want to do.

如果存在8个可用线程，

If there exist 8 available threads,

job(1)和job(2)必须首先同时具有7个线程和1个线程同时运行.

job(1) and job(2) have to run simultaneously at first with 7 threads and 1 thread, respectively.

在job(2)完成之后，应该将job(2)使用的线程分配给并行于for循环的job(1).

After job(2) finishes, the thread that job(2) was using should be allocated to job(1) which is the parallel for loop.

我正在使用

I'm using omp_get_thread_num to count how many threads are active in each region. I would expect the the number of threads in job(1) increases by 1 when job(2) finishes.

以下描述了可能是错误或确定的解决方案:

Below describes a solution that might be wrong or ok:

  omp_set_nested(1);
  #pragma omp parallel
  {
    #pragma omp sections
    {
      #pragma omp section // job(2)
      { // 'printf' is not real job. It is just used for simplicity.
        printf("i'm single: %d\n", omp_get_thread_num());
      }
      #pragma omp section // job(1)
      {
        #pragma omp parallel for schedule(dynamic, 32)
        for (int i = 0 ; i < 10000000; ++i) {
          // 'printf' is not real job. It is just used for simplicity.
          printf("%d\n", omp_get_thread_num());
        }
      }
    }
  }

如何使我想要完成的工作完成?

How can make the work that I want to achieve be done?

推荐答案

这样的事情怎么办?

#pragma omp parallel
{
     // note the nowait here so that other threads jump directly to the for loop
    #pragma omp single nowait
    {
       job2();
    }

    #pragma omp for schedule(dynamic, 32)
    for (int i = 0 ; i < 10000000; ++i) {
        job1();
    }
}

我没有对此进行测试，但是由于nowait，单个线程将仅由一个线程执行，而所有其他线程将直接跳至for循环. 另外，我认为它比章节更容易阅读.

I did not test this but the single will be executed by only one threads while all others will jump directly to the for loop thanks to the nowait. Also I think it is easier to read than with sections.

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